1. Field of the Invention
The invention relates to an amplifier circuit, and more particularly, to an amplifier circuit having a compensating amplifier unit for improving loop gain and reducing distortion.
2. Description of the Prior Art
The negative feedback mechanism that is capable of improving system linearity is broadly applied in the electronics industry. Please refer to FIG. 1. FIG. 1 is a schematic diagram illustrating a conventional feedback system 100. The feedback system 100 can be achieved utilizing an open-loop gain A(s) and a feedback factor β. As one of ordinary skill in the pertinent art will understand, the relationship between an output voltage Vo and an input voltage Vi of the feedback system 100 is shown as follows:
                              Vo          Vi                =                              A            ⁡                          (              s              )                                            1            +                                          A                ⁡                                  (                  s                  )                                            ⁢              β                                                          Formula        ⁢                                  ⁢        1            
Oftentimes, when the value of the loop gain A(s)β is much larger than 1, the relationship between the output voltage Vo and the input voltage Vi of the feedback system 100 can be simplified as follows in Formula 2:
                                          Vo            Vi                    =                                    1              β                        ⁢                                                  ⁢            as            ⁢                                                  ⁢                          A              ⁡                              (                s                )                                      ⁢            β                          >>        1                            Formula        ⁢                                  ⁢        2            
The feedback factor, β, is a frequency irrelevant term. It means that Vo is proportional to Vi in all frequencies and the gain is a constant,
  1  βif only if the loop gain A(s)β is much larger than 1. However, if the feedback factor β is increased, the close-loop gain
  1  βis inevitably decreasing. Given this relationship, the feedback factor β cannot be increased in an unlimited or unbounded fashion as a method for increasing the loop gain A(s)β of the feedback system 100. Therefore, one of the practical means for increasing the loop gain A(S)β of the feedback system 100 is by increasing the open-loop gain A(s).
Please refer to FIG. 2. FIG. 2 is a circuit diagram of a typical amplifier circuit 200. The amplifier circuit 200 includes an integrator circuit 210, an output stage amplifier unit 220, and a feedback resistor Rf. The integrator circuit 210 includes an operational amplifier 212, a resistor R1, and a capacitor C1. In the amplifier circuit 200, the output stage amplifier unit 220 is commonly realized as an output stage with stable and fixed gain G, and the value of the gain G is designed according to various operational requirements and noise-rejection limitation. Generally, the value of the gain G is not large enough to serve as an open loop gain stage along. In order to increase the open-loop gain A(s) in an effort to improve the system linearity, the method of related art will increase the open-loop gain by A(s) coupling the integrator circuit 210 to the output stage amplifier unit 220. Adding the integrator circuit 210 not only increases the open-loop gain A(s) but also serves the additional purpose of filtering unnecessary high-frequency noise. As one of ordinary skill in the pertinent art will understand, the relationship between an output voltage Vo and an input voltage Vi of the amplifier circuit 200 is shown as follows:
                              Vo          Vi                =                                            -                              G                                                      sC                    1                                    ⁢                                      R                    1                                                                                      1              +                              G                                                      sC                    1                                    ⁢                                      R                    f                                                                                =                                                    -                                  G                                                            sC                      1                                        ⁢                                          R                      1                                                                                                  1                +                                                      (                                          -                                              G                                                                              sC                            1                                                    ⁢                                                      R                            1                                                                                                                )                                    ⁢                                      (                                          -                                                                        R                          1                                                                          R                          f                                                                                      )                                                                        =                                          A                ⁡                                  (                  s                  )                                                            1                +                                                      A                    ⁡                                          (                      s                      )                                                        ⁢                  β                                                                                        Formula        ⁢                                  ⁢        3            
wherein
                                          A            ⁡                          (              s              )                                =                      -                          G                                                sC                  1                                ⁢                                  R                  1                                                                    ,                  β          =                      -                                          R                1                                            R                f                                                                        Formula        ⁢                                  ⁢        4            Additionally, the loop gain A(s)β of the amplifier circuit 200 can be detailed as shown below in Formula 5:
                                          A            ⁡                          (              s              )                                ⁢          β                =                  G                                    sC              1                        ⁢                          R              f                                                          Formula        ⁢                                  ⁢        5            
As shown in Formula 5, in the context of the related art, the loop gain A(s)β of the amplifier circuit 200 can be increased to improve the system linearity by increasing the gain G of the output stage amplifier unit 220 or by either decreasing the capacitor C1 of the integrator circuit 210 or decreasing the resistance value of feedback resistor Rf. As mentioned previously, the gain G is usually small. And because of system stability issues and limitations of the operational frequency range, the capacitor C1 of the integrator circuit 210 or the resistance of feedback resistor Rf cannot be decreased unlimitedly or in an unbounded fashion. In consideration of these factors, the loop gain A(s)β of the prior art is limited to a certain level and the linearity of the system can't not be further improved.